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All answers having "Observer" as clue:

4 Character Answers:

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EYER

SEER



5 Character Answers:

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NOTER

SCOUT



6 Character Answers:

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HEEDER

STARER

VIEWER



7 Character Answers:

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NOTICER

WITNESS



8 Character Answers:

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ONLOOKER



9 Character Answers:

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SPECTATOR



10 Character Answers:

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EYEWITNESS